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I’ve had a fiber line for residential internet buried in my yard. At one point it stopped working and a tech was able to pinpoint the exact location that it had

I’ve had a fiber line for residential internet buried in my yard. At one point it stopped working and a tech was able to pinpoint the exact location that it had a slight kink using a device that measured the reflection. They just dug up that section with hand tools to unkink it.

mlyle 3y ago
Yup-- an optical time domain reflectometer. Though TDRs are in common use for copper cable, too. You send the pulse down the line and get a reflection back. They're a little worse than optical ones because there's more uncertainty of the speed pulses travel down a random network cable than a piece of optical fiber.
- I_Am_Nous 3y ago
  They make some really cool in-line OTDRs these days, but the one we use was about $10k I believe and it's had a broken screen for years. Still works if you plug a mouse in!
- hunter2_ 3y ago
  Does varying twist rate contribute to that uncertainty? I imagine that a cable with a high twist rate is slower because the twist means the individual conductors are longer (maybe just by a couple percent) than one with a lower twist rate. And if the test equipment isn't configured for the pinout scheme being used (A vs B) then it might not even be pulsing a single pair at a time (each pair has a different twist rate) although perhaps that issue is automatically avoided.
  - mlyle 3y ago
    > Does varying twist rate contribute to that uncertainty? Yes, but beyond the effect that you're saying. The distance between conductors affects speed down the cable. Tighter twisting also changes the distance and the amount of dielectric around, which also varies speed of propagation. These effects can be massive; a typical cat 7 cable has a velocity factor of 80% of the speed of light, varying by a couple of percent; a cat 5e cable more is often more like 65%. If you launch a pulse into a mess of mixed wiring to find out where something is unplugged or cut, you could be off by 20% or more.
    - deleted 3y ago
      [deleted]
shpx 3y ago
How does a cable develop a kink while it's buried?
- isaacdl 3y ago
  I don’t know, but it happened relatively quickly (within a few days) of initial installation. I suspect either settling soil or animal activity.
  - type0 3y ago
    > or animal activity more likely tree roots hugging it

mlyle 3y ago

Yup-- an optical time domain reflectometer. Though TDRs are in common use for copper cable, too. You send the pulse down the line and get a reflection back. They're a little worse than optical ones because there's more uncertainty of the speed pulses travel down a random network cable than a piece of optical fiber.

I_Am_Nous 3y ago
They make some really cool in-line OTDRs these days, but the one we use was about $10k I believe and it's had a broken screen for years. Still works if you plug a mouse in!
hunter2_ 3y ago
Does varying twist rate contribute to that uncertainty? I imagine that a cable with a high twist rate is slower because the twist means the individual conductors are longer (maybe just by a couple percent) than one with a lower twist rate. And if the test equipment isn't configured for the pinout scheme being used (A vs B) then it might not even be pulsing a single pair at a time (each pair has a different twist rate) although perhaps that issue is automatically avoided.
- mlyle 3y ago
  > Does varying twist rate contribute to that uncertainty? Yes, but beyond the effect that you're saying. The distance between conductors affects speed down the cable. Tighter twisting also changes the distance and the amount of dielectric around, which also varies speed of propagation. These effects can be massive; a typical cat 7 cable has a velocity factor of 80% of the speed of light, varying by a couple of percent; a cat 5e cable more is often more like 65%. If you launch a pulse into a mess of mixed wiring to find out where something is unplugged or cut, you could be off by 20% or more.
  - deleted 3y ago
    [deleted]

shpx 3y ago

How does a cable develop a kink while it's buried?

isaacdl 3y ago
I don’t know, but it happened relatively quickly (within a few days) of initial installation. I suspect either settling soil or animal activity.
- type0 3y ago
  > or animal activity more likely tree roots hugging it